Potassium contracture and calcium influx in frog's skeletal muscle

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Abstract
The height and the duration of the K contracture of frog's sartorii ( Rana esculenta) is a function of extracellular calcium. The rate of inhibition of the K contracture in a Ca-free solution and the rate of recovery from such inhibition in a Ca-containing solution measured by the percentage increase and decrease of the height of the K contracture are not alike, the recovery being quicker. The apparent diffusion constant of Ca as calculated from recovery data is about ten times, in Na-free solutions about 100 times, smaller than that calculated from inhibition data. The additional Ca influx connected with contractures, measured by Ca45 entry, is almost proportional to the extracellular Ca concentration. However, the size of K contractures of and the Ca influx into muscles soaked in Ca-free nitrate solutions suggests that both the "removable" as well as the "more firmly surface-bound" calcium take part in the K contracture.